This was already requested by @KuroSensei in #627 back in 2020, specifically because --fixed-strings performs substring matching even when undesirable, and there's no way to turn that off. @sharkdp suggested that --glob can be used instead and closed the issue, but I say that this is not a good solution because globbing syntax can interfere and cause undesirable behavior.

--exact would perform --fixed-strings without substring matching.